High density magnetic random access memory

ABSTRACT

A magnetic memory device that comprises a substrate, a memory cell including a magnetic tunnel junction which comprises a free ferromagnetic layer having a reversible magnetization direction directed perpendicular to the substrate, a pinned ferromagnetic layer having a fixed magnetization direction directed perpendicular to the substrate, and an insulating tunnel barrier layer disposed between the pinned and free layers, a first electrical circuit for applying a first current to a first conductor electrically coupled to the free layer to produce a bias magnetic field along a hard axis of the free layer, a second electrical circuit for applying a second current to a second conductor electrically coupled to the pinned layer to cause a spin momentum transfer in the free layer, wherein magnitudes of the bias magnetic field and spin momentum transfer in combination exceed a threshold and thus reverse the magnetization direction of the free layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of provisional patent applicationNo. 61/472,788, filed on Apr. 7, 2011 by the present inventors.

FEDERALLY SPONSORED RESEARCH

Not Applicable

SEQUENCE LISTING OR PROGRAM

Not Applicable

RELEVANT PRIOR ART

U.S. Pat. No. 5,640,343, Jun. 17, 1997—Gallagher et al.

U.S. Pat. No. 7,224,601, May 29, 2007—Panchula

U.S. Pat. No. 7,529,121, May 5, 2009—Kitagawa et al.

BACKGROUND

Magnetic random access memory (MRAM) using spin-induced switching is astrong candidate for providing a dense and fast non-volatile storagesolution for future memory applications. Each MRAM includes an array ofmemory cells. FIG. 1 shows a schematic view of MRAM cell employing aspin-induced writing mechanism according to a prior art. The cellcomprises a magnetoresistive element (or magnetic tunnel junction) J, aselection transistor T, a bit line BL, a word line WL, and a source lineSL. The bit and word lines are formed in different layers and intersecteach other in space. The magnetoresistive (MR) element J and theselection transistor T are connected in series and disposed in avertical space between intersecting bit and word lines. They areconnected to the source line SL at one end and to the bit line BL atanother end. The word line is connected to a gate terminal of theselection transistor T. The MR element J comprises at least a pinned (orreference) layer 12 with a fixed direction of magnetization (shown by asolid arrow), a free (or storage) layer 16 with a reversiblemagnetization direction (shown by a dashed arrow), and a tunnel barrierlayer 14 disposed between the pinned and free magnetic layers. Thedirection of the magnetization in the free layer 16 can be controlled bya direction of a spin-polarized current I_(S) running through theelement J in a direction perpendicular to a film surface. Resistance ofthe MR element depends on a mutual orientation of the magnetizations inthe magnetic layers 12 and 16. The resistance is low when themagnetizations in the layers 12 and 16 are parallel to each other (logic“0”), and high when the magnetizations are antiparallel (logic “1”).Difference in the resistance between two magnetic states can exceedseveral hundred percent at room temperature.

FIG. 2 shows a circuit diagram of a portion of MRAM 20 with spin-inducedswitching according to a prior art. The MRAM 20 includes an array 22 ofmemory cells C11-C33 (other cells are not shown) disposed in a verticalspace between pluralities of parallel bits and word lines at theirintersections. Each memory cell comprises an MR element J and transistorT connected in series. A plurality of parallel bit lines BL1-BL3 isconnected to a bit line driver 24. A plurality of the word lines WL1-WL3is connected to a word line driver 26. A plurality of the parallelsource lines SL1-SL3 is connected to a source line driver 28. Selectionof a memory cell in the array 22 is provided by applying a suitablesignal to appropriate bit and word lines. For instance, to select thememory cell C22 that is located at the intersection of the bit line BL2and the word line WL2, the signals need to be applied to these linesthrough the drivers 24 and 26, respectively.

Cell size is one of key parameters of the MRAM. It substantially dependson the size and number of selection transistors supplying aspin-polarized write current to a MR element. The number of thetransistors controlling the write current usually vary from one to twoper a MR element. It depends on a saturation current of a selectiontransistor and magnitude of the spin-polarized current required to causeswitching of the MR element. Frequently, especially for MR elementshaving in-plane magnetization in magnetic layers, one selectiontransistor cannot provide the required spin-polarized current due to itssaturation. This obstacle prevents the MRAM cell size reduction.

Another important parameter of MRAM is a write speed. The write speeddepends on a magnitude of the spin-polarized current running through theMR element. High speed (short duration of the write current pulse)requires higher magnitude of the spin-polarized current that can belimited by the saturation current of the selection transistor or by abreakdown of the tunnel barrier layer.

The present disclosure addresses to the above problems.

SUMMARY

Disclosed herein is a magnetic memory device that comprises a substrate,a memory cell including a magnetic tunnel junction which comprises afree ferromagnetic layer having a reversible magnetization directiondirected substantially perpendicular to the substrate in an equilibriumstate, a pinned ferromagnetic layer having a fixed magnetizationdirection, and an insulating tunnel barrier layer disposed between thepinned ferromagnetic layer and the free ferromagnetic layer, a firstelectrical circuit for applying a first current to a first conductorcomprising ferromagnetic cladding to produce a bias magnetic fieldapplied along a hard magnetic axis of the free ferromagnetic layer, thefirst conductor is electrically coupled to the free ferromagnetic layer,a second electrical circuit for applying a second current to a secondconductor to cause a spin momentum transfer in the free ferromagneticlayer, the second conductor is electrically coupled to the pinnedferromagnetic layer, wherein a magnitude of the bias magnetic field anda magnitude of the spin momentum transfer in combination exceed athreshold and thus reverse the magnetization direction of the freeferromagnetic layer when the first write current and the second writecurrent are applied to the memory cell at the same time.

Also disclosed a magnetic memory device that comprises a substrate, afirst plurality of electrically conductive lines formed on thesubstrate, a second plurality of electrically conductive lines formed onthe substrate and overlapping the first plurality of lines at aplurality of intersection regions, a plurality of memory cells formed onthe substrate and arranged in an array, each memory cell being locatedat an intersection region and comprising a magnetic tunnel junctionwhich includes a free ferromagnetic layer having a reversiblemagnetization direction directed substantially perpendicular to thesubstrate in an equilibrium state, a pinned ferromagnetic layer having afixed magnetization direction, and an insulating tunnel layer disposedbetween the pinned ferromagnetic layer and the free ferromagnetic layer,each magnetic tunnel junction is electrically coupled to one of thefirst plurality of lines at the free ferromagnetic layer and to one ofthe second plurality of lines at the pinned ferromagnetic layer, whereinthe magnetization direction of the free ferromagnetic layer is reversedby a spin-polarized current flowing through the magnetic tunnel junctionin a direction perpendicular to the substrate.

Also disclosed a method for writing to a magnetic memory device thatincludes a plurality of magnetic tunnel junctions formed on a substrateand arranged in columns and rows, each magnetic tunnel junctioncomprising a free ferromagnetic layer having a reversible magnetizationdirection directed substantially perpendicular to the substrate in anequilibrium state, a pinned ferromagnetic layer having a fixedmagnetization direction, and an insulating tunnel barrier layer disposedbetween the free ferromagnetic layer and the pinned ferromagnetic layer,the method includes applying a bias current to a bit conductive linecomprising ferromagnetic cladding and being electrically coupled to arow of magnetic tunnel junctions at the free ferromagnetic layer toproduce a bias magnetic field along a hard magnetic axis of the freeferromagnetic layer, applying a first current to a conductive word lineelectrically coupled to a column of magnetic tunnel junctions at thepinned ferromagnetic layer to produce a spin momentum transfer in thefree ferromagnetic layer of a magnetic tunnel junction located at afirst intersection region of the bit conductive line and the wordconductive line, wherein the bias current and the first current areapplied at the same time, and a joint effect of the bias magnetic fieldand the spin momentum transfer causes a reversal of the magnetizationdirection of the free ferromagnetic layer of the magnetic tunneljunction located at the first intersection region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a memory cell with spin-induced switchingaccording to a prior art.

FIG. 2 is a circuit diagram of a magnetic random access memory withspin-induced switching according to a prior art.

FIGS. 3A and 3B is a circuit diagram of magnetic random access memorywith a spin-induced switching according to an embodiment of the presentdisclosure illustrating writing of logic “0” and logic “1” to a memorycell.

FIG. 4 is a schematic view of a memory cell with a spin-inducedswitching according to an embodiment of the present disclosure.

FIG. 5 is a schematic view of a memory cell with a hybrid switchingmechanism.

FIG. 6A is a circuit diagram of magnetic random access memory withhybrid switching mechanism illustrating writing a logic “0” to a memorycell according to another embodiment of the present disclosure.

FIG. 6B is a circuit diagram of magnetic random access memory withhybrid switching mechanism illustrating writing logic “1” to severalmemory cells simultaneously according to another embodiment of thepresent disclosure.

FIG. 7 is a circuit diagram of the magnetic random access memory shownin FIG. 6A during a read operation.

EXPLANATION OF REFERENCE NUMERALS

12 pinned (or reference) magnetic layer

14 tunnel barrier layer

16 free (or storage) magnetic layer

20,30, 60 magnetic random access memory (MRAM)

22 array of memory cells

24 bit line driver

26 word line driver

28 source line driver

52 conductor

54 magnetic flux concentrator

56 non-magnetic gap

BL, BL1, BL2, BL3 bit line

C11-C33 memory cell

J, J11-J33 magnetic tunnel junction

SA1-SA3 sense amplifier

SL, SL1, SL2, SL3 source line

T, T11-T33 selection transistor

Tb1-Tb6 bit line transistor

Ts1-Ts3 read transistor

Tw1-Tw6 word line transistor

WL, WL1, WL2, WL3 word line

DETAILED DESCRIPTION

Embodiments of the present disclosure will be explained below withreference to the accompanying drawings. Note that in the followingexplanation the same reference numerals denote constituent elementshaving almost the same functions and arrangements, and a repetitiveexplanation will be made only when necessary.

Note also that each embodiment to be presented below merely discloses andevice or method for embodying the technical idea of the presentdisclosure. Therefore, the technical idea of the present disclosure doesnot limit the materials, structures, arrangements, and the like ofconstituent parts to those described below. The technical idea of thepresent disclosure can be variously changed within the scope of theappended claims.

Refer now to the drawings, FIG. 1, FIG. 4, and FIG. 5 illustrateexemplary aspects of MR element. Specifically, these figures illustratethe MR element having a multilayer structure with a perpendiculardirection of magnetization in magnetic layers. The direction (ororientation) of the magnetization in the magnetic layers are shown bysolid or dashed arrows. The magnetization in the magnetic layer can bedirected perpendicular or in-plane to surface of the magnetic layers.The MR element can store binary data by using steady logic statesdetermined by mutual orientation of the magnetizations in the magneticlayers separated by a tunnel barrier layer. The logic state “0” or “1”of the MR element can be changed by a spin-polarized current runningthrough the element in the direction across the tunnel barrier layer orperpendicular to a film surface.

The MR element herein mentioned in this specification and in the scopeof claims is a general term of a tunneling magnetoresistance (TMR)element using an insulator or semiconductor as the tunnel barrier layer.Although the above mentioned figures each illustrate the majorcomponents of the MR element, another layer (or layers) such as a seedlayer, a pinning layer a cap layers, and others may also be included.

FIGS. 3A and 3B show a circuit diagram of a portion of MRAM 30 accordingto an embodiment of the present disclosure. The memory includes an array22 of memory cells C11-C33, a plurality of parallel bit lines BL1-BL3connected at their end to a bit line driver 24, and a plurality ofparallel word lines WL1-WL3 connected at their end to word line driver26.

Each memory cell comprises an MR element without a selection transistor.The MR element is connected to the appropriate bit and word lines at itsends and disposed at the intersection of the lines in a vertical spacebetween them. Schematic view of the memory cell of the MRAM 30 is shownon FIG. 4. The MR element J comprises at least a pinned magnetic layer12 having a fixed magnetization direction (shown by a solid arrow), afree magnetic layer 16 having a variable (or reversible) magnetizationdirection (shown by a dashed arrow), and a tunnel barrier layer 14disposed between the pinned and free magnetic layers. The free magneticlayer 16 can be made of a magnetic material with a substantialspin-polarization and has a the magnetization directed substantiallyperpendicular to a layer surface in its equilibrium state. For example,the free magnetic layer 16 can be made of (Co₃₀Fe₇₀)₈₅B₁₅ (% atomic)alloy having a thickness of about 1.5 nm. The pinned magnetic layer 12can be made of a magnetic material with a substantial spin-polarizationand has the magnetization directed substantially perpendicular to alayer surface. For example, the pinned magnetic layer can be made of the(Co₃₀Fe₇₀)₈₅B₁₅ (% atomic) alloy having a thickness of about 2.5 nm. Thetunnel barrier layer 14 can be made of MgO having a thickness of about1.1 nm. The free, tunnel barrier and pinned layers form a substantiallycoherent texture having a BCC (body-centered cubic) structure with (001)plane orientation. The MR element with this crystalline structureprovides a substantial tunneling magnetoresistance (TMR≧100% at roomtemperature) and a density of spin-polarized write current of about1·10⁶ A/cm² or less. These parameters are essential for MRAM.

In the MRAM 30 shown in FIGS. 3A and 3B the pluralities of theconductive bit and the word lines intersect each other but spaced fromeach other in direction perpendicular to a plane of substrate (notshown). Each of the memory cells C11-C33 comprises an appropriate MRelement J11-J33 that is disposed at an intersection of a bit and wordline in the vertical space between them. The MR element is electricallyconnected to the intersecting bit and the word lines by its oppositeends. For instance the memory cell C22 comprises the MR element J22disposed at the intersection of the bit line BL2 and the word line WL2.The MR element J22 is electrically connected to the word line WL2 at itsfirst end and to the bit line BL2 at its second end.

The bit lines BL1-BL3 extend in an X-direction. They are electricallyconnected at one end to a bit line driver 24 that includes CMOStransistors Tb1-Tb6. For example, the bit line BL2 is connected by oneend to a common drain terminal formed by a n-type transistor Tb3 andp-type transistor Tb4. A source terminal of the p-type transistor Tb4 isconnected to a power supply. A source terminal of the n-type transistorTb3 is connected to a ground. Similarly the bit lines BL1 and BL3 areconnected to the pairs of CMOS transistors Tb1, Tb2 and Tb5, Tb6,respectively. Gate terminals of the transistors Tb1-Tb6 are connected tothe bit line driver 24. The bit line driver 24 operates as a rowselection switch.

The word line WL1-WL3 extend in an Y-direction crossing the X-direction.One end of the word line WL1-WL3 is connected to the word line driver26. The driver 26 comprises a plurality of read/write circuits. Each ofthe read/write circuits includes at least a pair of CMOS transistorscomprising one of p-type transistors Tw2, Tw4 or Tw6 and one of n-typetransistors Tw1, Tw3 or Tw5 connected in series to each other, and asense amplifier SA1-SA3. Each of the transistors pairs Tw1 and Tw2, Tw3and Tw4, Tw5 and Tw6 is connected to a power supply at a source terminalof the appropriate p-type transistor and to the ground at a sourceterminal of the appropriate n-type transistor. The word line isconnected to a common drain terminal of the CMOS transistor pair and toone input terminal of the sense amplifier SA through a read transistorTs. For example, the word line WL2 is connected by its end to the commondrain terminal formed by the transistor Tw3 and Tw4 and to the firstinput terminal of the sense amplifier SA2 through the read transistorTs2. Second input terminal of the sense amplifier SA2 is connected to areference element (not shown). Gates of the transistors Tw1-Tw6 areconnected to the word line driver 26. The driver 26 operates as a columnselection switch.

The sense amplifier SA1-SA3 comprises at least two inputs. One input ofthe amplifier is connected to the end of the word line WL1-WL3 and tothe common drain terminal of the transistor pair by mean of the readtransistor Ts1-Ts3. The other input of the sense amplifier is connectedto a reference element (not shown). The sense amplifier judges a datavalue of the MR element inside of the selected memory cell based on areference signal Ref.

The memory 30 shown in FIGS. 3A and 3B comprises the array 22 of the MRelements J11-J33 disposed above the silicon wafer (not shown). Theselection transistors Tb1-Tb6 and Tw1-Tw6 may be positioned along aperimeter of the array 22. The wafer area located underneath of thememory array is not occupied by the selection transistors and can beused for another circuits. Hence the present design can provide asubstantial reduction of a chip/die size. Moreover, the peripherallocation of the selections transistors provides a possibility of usinglarge selection transistors or several transistors providing asubstantial write current that is essential for high speed writing.

The MRAM 30 shown in FIGS. 3A and 3B employs a spin-induced switchingmechanism of the MR elements. According to spin-induced switching theorientation of magnetization in the free layer 16 can be reversed by aspin-polarized current I_(S) running through the MR element (FIG. 4).Electrons of the write current have a substantial degree of spinpolarization that is predetermined by magnetic properties of the pinnedlayer 12. The spin-polarized electrons running through the free layer 16transfer a moment of their spins causing the magnetization in the freelayer to change its direction. Polarity of the magnetization in the freelayer 16 can be controlled by a direction of the spin-polarized currentI_(S) running through the MR element. The direction of thespin-polarized current in the MR element shown on FIG. 4 corresponds towriting a logic “0” or to parallel orientation of magnetizations in thefree 16 and pinned 12 magnetic layers.

FIG. 3A shows writing of a logic “0” to the MR element J22 of the memorycell C22. A switching current I_(S) is produced in the MR element byapplying appropriate input signals to the gate of the transistor Tb4(Write 0) and to the gate of the transistor Tw3 (Write 0). Bothtransistors are partially opened. The spin-polarized current I_(S) isrunning from the power supply through the transistor Tb4, bit line BL2,MR element J22, word line WL2, and transistor Tw3 to the ground. Theappropriate bit and word lines, and MR element are shown in bold. Forthe MR element having a configuration shown in FIG. 4 the current I_(S)is running from the free layer 16 to the pinned layer 12 through thetunnel barrier layer 14. The spin-polarized conductance electrons aremoving in opposite direction from the pinned layer 12 to the free layer16. For the giving direction of the current I_(S) the magnetization inthe free layer 16 will be directed in parallel to the magnetizationdirection of the pinned layer 12. This mutual orientation of themagnetizations corresponds to a low resistance state of the MR elementor to a logic “0”.

FIG. 3B illustrates writing logic “1” to the MR elements J22. The writecurrent I_(S) is supplied to the MR element J22 by simultaneouslyapplying an appropriate input signal to the gate of the transistors Tb3(Write 1) and Tw4 (Write 1). The transistors are partially opened andthe current I_(S) is running from the transistor Tw4 to the transistorTb3 through the word line WL2, MR element J22, and bit line BL2 (shownin bold). In the MR element J22 having a configuration shown in FIG. 4the spin-polarized current I_(S) is running from the pinned layer 12 tothe free layer 16. This direction of the spin-polarized current willorient the magnetization in the free layer 16 anti parallel to themagnetization direction of the pinned layer 12. This mutual orientationof the magnetizations corresponds to a high resistance state or to alogic “1”.

According to theory, the magnitude of the minimal spin-polarized currentthat is required to reverse the magnetization direction in the freelayer is given by

$\begin{matrix}{I_{C\; 0} = {{- \left( \frac{2\; e}{h} \right)}\frac{\alpha\; M_{S}V}{{g(\theta)}p}H_{EFF}}} & (1)\end{matrix}$where e is an electron charge, h is Plank constant, α is Gilbert'sdamping constant, M_(S) is saturation magnetization of the free layermaterial, V is volume of the free layer, and p is a spin polarization ofthe current. The factor g(θ) depends on the relative angle θ betweenvectors of magnetization (shown by arrows in FIG. 4) in the pinned 12 anfree 16 layers. The value of the factor g(θ) is minimal and close tozero when the vectors of the magnetizations in the free and pinnedlayers are parallel or anti parallel to each other (θ is equal to 0 or180 degrees). The factor g(θ) has its maximum value when the vectors ofmagnetizations in the layers are perpendicular to each other (the angleθ is equal to 90 or 270 degrees). Effective magnetic field H_(EFF)acting on the free layer depends on a direction of magnetization(in-plane or perpendicular) in the pinned and free layers. The effectivefield is given by the following equations for the in-plane and forperpendicular magnetic materials, respectively:H _(EFF//) =H _(K//)+2πM _(S) +H _(APP) +H _(DIP)   (2)H _(EFF⊥) =H _(K⊥), −4πM _(S) +H _(APP) +H _(DIP),   (3)where H_(K//) and H_(K⊥), is a field of uniaxial crystalline anisotropyof in-plane and perpendicular magnetic material, respectively; H_(APP)and H_(DIP) are applied external field and the dipole field from thepinned layer acting on the free layer. The factor −4πM_(S) arises fromthe demagnetizing field of the thin film geometry of the free layerhaving the perpendicular anisotropy. The same factor for the free layerwith in-plane anisotropy is equal to +2πM_(S). Hence, the MTJ withperpendicular anisotropy may require substantially smaller (depends onH_(K) and M_(S)) switching current than that with similar parameters buthaving the in-plane anisotropy.

The direction of the magnetization in the free layer 16 of the MRelement in its equilibrium states can be parallel or anti-parallel tothe magnetization direction in the pinned layer. At these conditions theswitching current that is required to reverse the magnetization in thefree layer has its maximum value. Moreover, the magnitude of the currentdepends significantly on the duration of a current pulse. The magnitudeof the switching current is almost inverse proportional to the pulseduration. Hence, the high speed writing (short current pulse) requireshigh switching current. Magnitude of the switching current is limited bythe probability of a tunnel barrier layer breakdown. The above obstacleslimit switching speed and endurance of MRAM with spin-induced switching.

The equation (1) suggests that the spin-polarized write current can bereduced significantly by changing the angle θ between the vectors of themagnetization in the free and pinned layers. Since the orientation ofmagnetization in the pinned layer 12 is fixed, the angle θ can bechanged by tilting the magnetization in the free layer 16 from itsequilibrium state. Tilt of the magnetization of the free layer 16 can beprovided by applying a bias magnetic field along a hard magnetic axis ofthe free layer 16.

FIG. 5 shows a schematic view of the memory cell comprising an MRelement with perpendicular magnetization in the pinned 12 and free 16magnetic layers along with adjacent bit BL and word WL lines. Inaddition to the spin-polarized switching current I_(S) a bias currentI_(B) is further supplied to the bit line BL. The bias current I_(B)running through the bit line BL produces a bias magnetic field H_(B)(shown by arrow) that is applied along the hard axis of the free layer16. To increase the bias magnetic field locally, in vicinity of the MRelement to further reduce the required bias current I_(B), the bit lineBL comprises a conductive wire 52 and a magnetic flux concentrator(magnetic flux cladding) 54. The magnetic flux concentrator 54 is madeof a soft magnetic material having a high permeability and a lowcoercivity such as NiFe. The flux concentrator 54 comprises anon-magnetic gap 56 formed on a side of the bit line BL facing the MRelement. The free layer 16 is disposed adjacent to the non-magnetic gap56 where the bias magnetic field H_(B) has a maximum. Additional layers,such as a seed layer can be placed between the free layer 16 and the bitline BL. Insertion of the additional layer (or layers) between the freemagnetic layer 16 and the bit line BL results in a reduction of the biasfield. The magnetic field H_(B) decreases almost inverse proportionallywith a distance between the free layer 16 and the bit line surfacecontaining the non-magnetic gap 56. FIG. 5 illustrates one exemplaryimplementation where a magnetic cladding is wrapped around a bit linethat carries the bias current. Other magnetic flux cladding designs mayalso be used. The magnetic flux cladding can be used for a word line aswell.

The bias magnetic field H_(B) generated by the bias current I_(B) isproportional to the current. For example, the current of 0.1 mA cangenerate a bias magnetic field of about 10 Oe in the vicinity of the MRelement made with 65 nm technology node. The magnitude of the bias fieldH_(B) is not sufficient to cause an unwanted reverse of themagnetization in the memory cells exposed to the bias field. Thereversal of the magnetization can be achieved when both the biasmagnetic field and spin-polarized current affect the MR elementsimultaneously. Hence the proposed hybrid writing mechanism provides agood selectivity of the memory cell in the array and significantreduction of the spin-polarized current I_(S). That is important forachieving a high endurance of MRAM operating at high speed, especially.

FIGS. 6A and 6B show a circuit diagram of a portion of MRAM 60 employinga hybrid write mechanism. The memory 60 comprises two bit line drivers24 connected to the opposite ends of the of the bit lines BL1-BL3. Theword lines WL1-WL3 are connected at one end to the word line drivers 26.

To write a logic “0” to the MR element J22 (FIG. 6A) a bias currentI_(B) is supplied to the bit line BL2 by applying appropriate inputsignal to the gate of transistor Tb3 (Write 0) and to the gate of thetransistor Tb4 (Write 0). The bias current I_(B) running through the bitline BL2 produces a bias magnetic field that is applied along the hardaxis of the free layer. The bias field causes a tilt of themagnetization vector in the free layer from its equilibrium state thatis perpendicular to the film surface. The magnitude and duration of thebias magnetic field can be controlled effectively by the input signal“Write 0” and “Write 0” applied to the gate of the transistor Tb3 andTb4. The bias current I_(B) alone cannot cause a reversal of themagnetization in the MR element J22 and adjacent the bit line BL2elements J21 and J23. Switching of the magnetization in the free layeris a joint effect of the bias magnetic field and a spin momentumtransfer of polarized electrons of the current I_(S) running through theMR element. To cause switching a spin-polarized current I_(S) issupplied to the MR element J22. The current I_(S) is running from thetransistors Tb3 to the transistor Tw3 through the MR element J22 locatedat the intersection of the bit line BL2 and word line WL2 (shown inbold). Simultaneous effect of the bias magnetic field and spin-polarizedcurrent results in a logic state reversal of the MR element J22.

The input signals applied to the gate of the transistors Tb3, Tb4, andTw4 should be synchronized in time. Pulses of the currents I_(B) andI_(S) can overlap each other partially (shifted in time) or completely.Order of the pulses at partial overlapping can be any. The transistorTb4 should be opened while any of the transistors Tb3 or Tw4 are opened.

The memory 60 also provides a possibility of simultaneous writing to theseveral MR elements having electrical contact with the energized bitline BL2 (FIG. 6B). The bias current is supplied to the bit line BL2 byapplying an appropriate input signal to the gate of the transistors Tb3(Write 1) and Tb4 (Write 1). The bias current I_(B) produces a biasmagnetic field along the entire line and tilts the direction of themagnetization in all MR elements adjacent to the bit line. This field isnot sufficient to cause a reversal of the magnetization directions inthe energized MR elements. To accomplish reversal a spin-polarizedcurrent needs to be applied to the element. FIG. 6B shows a circuitdiagram of a portion of memory 60 during writing logic “1” to the memorycells C22 and C23 simultaneously when a bias current is applied to theline BL2. The appropriate input signals “Write 1” are applied to thegate of the transistors Tw4 and Tw6 connected to the end of the wordlines WL2 and WL3, respectively. The MR elements J22 and J23 located atthe intersection of the word lines WL2 and WL3 with a bit line BL2 areexperienced to cumulative effect of the bias magnetic field produced bythe bas current I_(B) and spin-polarized current I_(S) running throughthe elements.

Data can be written to the memory cells C21, C22, C23 at the same timeby applying an appropriate signal to the gate of the transistors Tw1 orTw2, Tw3 or Tw4, Tw5 or Tw6. Simultaneous writing to several memorycells can provide significant reduction a write energy per bit by meansof more effective use of bias current.

Transistors Tb1-Tb6 connected to the bit lines BL1-BL3 and thetransistors Tw1-Tw6 connected word lines WL1-WL3 are experienced todifferent magnitudes of the current running through them during writing.Therefore they can have different saturation current that can beachieved by using different size of transistors or by using severaltransistors. For instance the transistors Tb1-Tb6 can have largersaturation current than the transistors Tw1-Tw6. The transistors Tw1-Tw6control the switching spin-polarized current in the MR elements of thearray 22.

FIG. 7 shows a circuit diagram of the memory 60 according in read modeof operation. To read the data stored in the memory cell C22 anappropriate input signal needs to be applied to the transistors Tb3(Read), Tw3 (Read), and Ts2 (Read). A signal produced by a read currentI_(R) running through the J22 represents a read signal that isproportional to a resistance of the MR element: high resistance for alogic “1” and a low voltage for the logic “0”. The read current I_(R) issmaller than the spin-polarized write current I_(S) and cannot cause thereverse of the magnetization in the free layer of the J22 especially dueto absence of the bias current I_(B). The read signal is applied to oneinput of a sense amplifier SA2 through the opened transistor Ts2. Areference read signal Ref from a reference memory cell (not shown) isapplied to another input of the sense amplifier SA2. An output of theamplifier SA2 provides an information about data stored in the memorycell C22.

The MR elements of the disclosed MRAMs can use magnetic materials within-plane and/or perpendicular direction of the magnetization.

While the specification of this disclosure contains many specifics,these should not be construed as limitations on the scope of thedisclosure or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments. Certain features that aredescribed in this specification in the context of separate embodimentscan also be implemented in combination in a single embodiment.Conversely, various features that are described in the context of asingle embodiment can also be implemented in multiple embodimentsseparately or in any suitable sub-combination. Moreover, althoughfeatures may be described above as acting in certain combinations andeven initially claimed as such, one or more features from a claimedcombination can in some cases be excised from the combination, and theclaimed combination may be directed to a sub-combination or variation ofa sub-combination.

It is understood that the above embodiments are intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the embodiments should be, therefore,determined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

While the disclosure has been described in terms of several exemplaryembodiments, those skilled in the art will recognize that the disclosurecan be practiced with modification within the spirit and scope of theappended claims. Specifically, one of ordinary skill in the art willunderstand that the drawings herein are meant to be illustrative, andthe spirit and scope of the disclosure are not limited to theembodiments and aspects disclosed herein but may be modified.

What is claimed is:
 1. A magnetic memory device comprising: a substrate;a first plurality of conductive lines disposed on the substrate; asecond plurality of conductive lines disposed on the substrate andoverlapping the first plurality of conductive lines at a plurality ofintersection regions; a plurality of magnetic tunnel junctions disposedon the substrate and arranged into an array, each magnetic tunneljunction being located at an intersection region and comprising a freeferromagnetic layer having a reversible magnetization direction directedsubstantially perpendicular to the substrate in an equilibrium state, apinned ferromagnetic layer having a fixed magnetization directiondirected perpendicular to the substrate, and a tunnel barrier layerdisposed between the pinned ferromagnetic layer and the freeferromagnetic layer, a magnetic tunnel junction being electricallycoupled to a conductive line of the first plurality of conductive linesat a first end adjacent to the free ferromagnetic layer and to aconductive line of the second plurality of conductive lines at a secondend adjacent to the pinned ferromagnetic layer; a first transistorcomprising a first saturation current and electrically coupled to theconductive line of the first plurality of conductive lines; and a secondtransistor comprising a second saturation current and electricallycoupled to the conductive line of the second plurality of conductivelines; wherein the first saturation current is substantially larger thanthe second saturation current; and wherein the first transistor and thesecond transistor are disposed along a perimeter of the array of themagnetic tunnel junctions.
 2. The magnetic memory device of claim 1,wherein the magnetization direction of the free ferromagnetic layer isreversed by a spin-polarized current running through the magnetic tunneljunction in a direction perpendicular to the substrate.
 3. The magneticmemory device of claim 2, wherein the direction of the spin-polarizedcurrent in the magnetic tunnel junction is reversible.
 4. The magneticmemory device of claim 1, wherein each conductive line of the firstplurality of conductive lines further comprising: a ferromagneticcladding disposed around the conductive line; and a nonmagnetic gapdisposed adjacent to the free ferromagnetic layer.